Error: Access to OData is disabled

When this error is encountered it likely means the Graph API isn't enabled for the Application you created for SigParser application you created in the Azure portal.

Here are some things to check.

Multiple regions for your Microsoft 365 deployments sync delay

We've seen issues before where customers who have global deployments of their Microsoft 365 will have a long sync delay for the mailbox access permissions. Depending on how often the environments sync it could take hours for the permissions to propagate. 

Our suggestion is to add the group members and check back the next day to see if the mailboxes could be scanned.

Administrative User

Certain users, we're not sure why, if they are administrators can't be mined by SigParser. Try using another user to test setting up the connection with.

ApplicationAccessPolicy not created

Did you create an ApplicationAccessPolicy by running the powershell command in the steps? Did you run the test steps to verify it was created properly?

Was the group you created a Mail Enabled Security Group?

You can't just make a normal group and assign it to the application. You need to make sure the group you defined was actually a Mail Enabled Security Group. 

ApplicationAccessPolicy using DenyAccess instead of RestrictAccess

When you ran the command to map the application to the group did you use DenyAccess instead of RestrictAccess. That might cause this error. See this article.